Entangled photon states are special quantum states of light which have been shown to be useful for various applications such as quantum key distribution and metrology. This invention is related to the creation of entangled photon states in a practical manner. Entangled light can be generated using various nonlinear processes including those in nonlinear crystals, such as described in U.S. Pat. No. 6,424,665 by P. G. Kwiat et al, as well as using the third order nonlinearity in fiber as described in U.S. Pat. No. 6,897,434 by Kumar et al. The entangled light is typically produced in two wavebands called the signal and idler bands, where the unique properties of entanglement only become apparent when measuring both wavebands simultaneously. The use of fiber is beneficial because it is often desired to send the entangled photons over long distances using low-loss optical fiber. By generating the entangled photons directly in fiber one can avoid coupling losses. Other benefits, such as very high spatial mode purity, are also realized.
Some schemes for realizing entanglement using the nonlinearity of fiber have been specified in U.S. Pat. No. 6,897,434 by Kumar et al. Later work was published as “Ultra Stable All-Fiber Telecom-Band Entangled Photon-Pair Source for Turnkey Quantum Communication Applications,” in Optics Express, 14, 6936, 2006 by Liang et al. which used a more advanced design in order to make the system more robust and easier to use. This so called “self-aligned” fiber-based entanglement source put the nonlinear fiber inside a Michelson interferometer with Faraday mirrors in order to make it insensitive to polarization fluctuations. Normally, interferometers need feedback control in order to stabilize phase and polarization fluctuations in the interferometer. This can be mitigated by using a Sagnac loop interferometer such that only a single polarization control element is needed to account for birefringence in the loop. A Sagnac loop is robust as it only needs a single polarization adjustment, but due to slow changes in the fiber birefringence it needs periodic re-alignment. The “self-aligned” scheme however, is supposed to be “alignment free” as it does not need any internal polarization control elements. Instead, the action of the Faraday mirror makes the system inherently stable.
Although this new design represents advancement in creating a robust entanglement source, it still has a number of drawbacks particularly as pertains to the complexity of the initial alignment of the source and subsequent detection apparatus. It is desirable to engineer an entangled photon source which is simple to align and for which the alignment of the source and the subsequent detection apparatus could be easily automated. The detection apparatus is a polarization analyzer, of which one implementation is shown in FIG. 1. The signal is input to one polarization analyzer 10 and the idler to another 11. They each contain a series of waveplates, in this case a half wave plate 12,13 and a quarter waveplate 14,15 although other types of polarization analyzers can use other components such as variable waveplates and have more or fewer components. In FIG. 1 each analyzer has a rotatable polarizer 16,17. The photons are detected with single photon detectors 18,19 after exiting the analyzer at which point the output from each detector is counted and correlated in a processor 20. It is the nature of entangled sources that interference can occur in the correlations of the detectors as a function of the angle of the rotatable polarizer, even though the statistics of the singles counts is not polarization dependent.
Since entangled light is effectively unpolarized, the photon counts from a particular detector are not a function of the setting of the polarization analyzer. However, the analyzer must be set properly in order to make a desired measurement. The settings may be relatively easy to determine when using an apparatus that generates entanglement in free-space. In such a case, as in U.S. Pat. No. 6,424,665 by P. G. Kwiat et al., the two orthogonal polarizations which are the constituent components of the entangled light exit the source, typically at polarizations called H and V, which can be referenced to the physical axis of the laboratory and correspond to horizontal and vertical polarizations. For this reason the polarization analyzer used in U.S. Pat. No. 6,424,665 is a simple half-wave plate followed by a polarizer which is equivalent to a rotatable polarizer. The H and V axis are clearly defined in physical space. There is a relative phase term between the H and V axis that must be set, producing an entangled state of |HH+eiφ|V|V, but that phase can be set, for instance, via changing the phase between the H and V axis on the pump wave. This phase will not drift considerably over time so the setting of the phase is a rare event.
Adjusting the polarization analyzer to the correct setting becomes more difficult if the entangled light propagates through fiber—particularly if the both the signal and idler propagate through different fibers as will generally be the case. This is because there is an unknown polarization rotation due to birefringence in the fiber. Physical space can no longer be used as a reference and the polarization rotation has two unknown degrees of freedom. One can not easily set the polarization analyzer using the entangled light directly. This is because the entangled light is not polarized so changing the analyzer settings has no effect on the singles counts. One can search for the settings that lead to the desired co-incidence count performance, but this is difficult to do due to the dimensionality of the system (two dimensions on two different detectors) and the fact that co-incidence counts are relatively rare events. Co-incidence counts are rare because losses reduce co-incidences in a quadratic way. If entanglement is distributed over a distance that causes 10 dB of loss and the effective detection efficiency of the single photon detectors are 10% (10 dB loss), then the co-incidence count rate is 1/100th of the singles count rate. Additionally, the number of photons generated in one measurement interval in an entangled state is typically much less than one. Thus, the overall co-incidence count rate is fairly low.
It would be beneficial if a polarized high-intensity source could be used to aid in alignment. This would allow one to produce many alignment photons per each measurement interval whereas the entangled state generation typically generates less than one photon per measurement interval. A higher photon rate allows for faster measurement speed and therefore faster alignment. The speed at which the system can be aligned is particularly important in fiber, since the birefringence in fiber changes as a function of time. Thus, being able to quickly determine the correct settings for the polarization analyzer is of importance.
U.S. Pat. No. 6,897,434 deals with the generation of entangled light in fiber. The invention details some methods of dealing with the aforementioned issue. In one method, the pump, signal, and idler wavelengths are all separated in the same free-space system. Thus, they all see similar birefringence in the fiber. Additionally, a signal is injected into the nonlinear fiber which then experiences amplification. This amplified signal can be used to monitor the relative phase between the H and V components of the pump. This phase is adjusted with another free space system to lock it to the desired value. The scheme works well and in a repeatable and systematic manner. However, it is limiting due to the fact that the signal and idler wavelengths propagate through the same fiber even though in general the goal of entanglement distribution is to separate the signal and idler. Additionally it is complex and many free-space components are used. Another design which uses fiber-based filters is also described in U.S. Pat. No. 6,897,434, however the alignment of the scheme would be much more difficult and time consuming since the monitor signal is no longer properly aligned with respect to the signal and idler entangled states. Additionally, all the methods described in this prior art work required substantial manual alignment of the source. Later prior art showed more robust designs, but an efficient method of quickly aligning the polarization analyzers is not described. It is desired to design a robust fiber-based entangled source that is simple to use and for which the polarization analyzers can be aligned quickly.
For applications requiring both entangled photon pairs to be generated in the most useful telecommunication wavelengths (1.3 or 1.55 micron light) Raman scattering limits the performance of fiber-generated entanglement. The Raman effect can be controlled by cooling the fiber, but it is practically desirable to attain similar performance at higher temperatures. In principle it is possible to reduce the Raman effect and thereby reduce or eliminate the cooling burden by spacing the two entangled photon wavelengths, called the signal and idler, suitably close together. However, filtering out the pump with an acceptable extinction becomes more and more difficult as the signal and idler wavelengths approach each other. Because the pump has a very large number of photons per measurement interval and the signal and idler entangled state has less than one photon per measurement interval, very high extinction ratios of the pump—typically on the order of 100 dB—may be required. Spacing the pump/signal/idler close together makes such filtering more difficult. The nonlinearity in the fiber interaction also causes self phase modulation of the pump, thereby expanding its spectrum and possibly contributing to deleterious cross talk.
The invention described here allows for closely spaced signal and idler wavelengths by proper design of both the pump spectrum and the receive filters. In particular, the use of a common narrow-band periodic filter, such as an etalon filter, in combination with standard telecommunication grade filters allows the entangled output to have a narrow bandwidth that is also easily isolated from the huge amount of unwanted pump photons. This configuration is an efficient and convenient method to space the signal and idler wavelengths close together in wavelength thereby reducing the deleterious Raman effect.
It is desirable to have pump/signal/idler filters with a close wavelength separation, a high extinction ratio, and a system design that generates an adequate number of entangled photon pairs. Also it is beneficial to have a system which is flexible, for instance one that can generate entangled pairs at different repetition rates, as well as in a small, robust, and inexpensive way. Pulsed systems are sometimes preferable to continuous wave systems, as it is easier to generate the high peak powers needed to generate nonlinearity in fiber with pulses, and some types of detectors work best in a time-gated mode.
U.S. Pat. No. 6,897,434 generated high quality entanglement (as measured by high visibility two-photon interference fringes) using signal and idler wavelengths which were separated 9 nm from the pump and a thin-film filter with a 0.5 nm filter bandwidth. In order to keep the same pair generation rate and keep self-phase modulation leakage from the pump to the signal the same, one can use the same peak power and reduce the filter bandwidth and wavelength separation in equal proportions. Let us use a figure of merit R=9 nm/0.5 nm=18 to represent the incumbent system wavelength separation-to-bandwidth ratio. All other things being equal, a higher R is better. A lower R may not work well due to SPM leakage and it may also make it difficult to get enough pump extinction. Since we want to bring the signal/idler wavelengths closer together to reduce Raman leakage, our goal is to reduce the wavelength separation as much as possible while still having a large R (>18). To allow for more convenient cooling or, better yet, room temperature operation, we will want to space the wavelengths much closer—say ˜1.5 nm or less—while still maintaining a high R parameter. If one can build better filters using some particular technology, such as thin film filters, that have the needed extinction ratios (˜100 dB) and low loss at the desired wavelength separation and the appropriate bandwidth (to maintain large R) while still allowing for a convenient laser to be used then one could envision a practical system with quality entanglement at much higher temperatures. Unfortunately, this has not been demonstrated yet because the various constraints are hard to simultaneously satisfy.
To have a large R parameter at a small wavelength separation one needs a relatively narrow filter. For instance, to have an R>18 at about 1.5 nm separation near 1550 nm one needs a filter bandwidth of ˜10 GHz. For reasons of stability and convenience (so that, for instance, the laser doesn't need complex frequency locking) it is also not desirable to have too narrow of a filter. As a rule of thumb we might guess that a bandwidth approximately within one order of magnitude of 4 GHz is desirable in the 1550 nm or 1310 nm bands. For instance, with a 1 nm wavelength separation at 1550 nm a 4 GHz filter bandwidth would lead to an R>30 which is better than prior art while at the same time also spacing the wavelengths much closer so as to substantially reduce Raman scattering.
Instead of filtering a wide bandwidth source to produce the pump signal, as was done in U.S. Pat. No. 6,897,434, the designated filter bandwidth range also allows for one to directly carve pulses using, for instance, Mach-Zehnder interferometric modulators or other optical modulators to create optical pulses which have bandwidths that inherently match the filters. It is likely one will also want to filter the carved output to eliminate spurious photons outside this band, however the extinction ratio requirements of the filter are greatly relaxed as opposed to filtering a source with a broad bandwidth that has significant power levels in the undesired signal and idler bands. Additionally it opens up more filtering options since the spectral profile of the pump will largely be determined by the temporal profile of the carved pulse. Note that the repetition rate of the pulses can easily be varied in such a scheme without changing the pulse bandwidth. Since repetition rate can be controlled with electronics, the same optical system is thus well suited for variable pulse rate operation.
The pump can thus be generated via pulse carving. This method is advantageous since the mode-locked lasers, which are usually used for generating high quality entanglement, are more expensive and have a larger footprint. While pulse carving has been used previously to generate entangled photons, (see H Takesue and K. Inone, “Generation of polarization-entangled photon pairs and violation of Bell's inequality,” Physical Review A 70, 031802 2004) it did not generate very good performance due to the lack of matched filtering. In this case, an external modulator was used to carve 100 ps pulses, the filters used arrayed waveguide gratings with bandwidths ˜25 GHz and the separation between the pump and signal wavelengths are about 400 GHz. The bandwidth of the pump pulse can be estimated to be about 4 GHz based on the temporal duration. Although the R parameter is reasonably good (400 GHz/25 GHz=8), the filters do not match the bandwidth of the pump pulses well at all (the filters are ˜25 GHz/4 GHz=6 times too wide). Also, the 400 GHz pump-to-signal separation is too far to allow for low-Raman effects at room temperature. Thus, although external pulse carving was used, the system design of the present invention was not implemented and therefore the desired goals were not met.
It is the object of the present invention to provide a fiber based entangled photon source that is robust, has a built-in method of generating a polarized signal for aiding in aligning the subsequent measurement apparatus, and can be integrated with an appropriate matched filtering scheme to allow for higher temperature operation. Polarization maintaining (PM) components, in some cases including a magneto-optical switching device, are judiciously employed in the invention.